Lesson 2: Sharing Data in Visual C#

Generator QR Code JIS X 0510 in Visual C# Lesson 2: Sharing Data

7. For the catch section of the try/catch block, catch a WaitHandleCannotBeOpenedException to determine that the named Mutex doesn t exist. 8. Next, test the Mutex variable created in step 2 for null (or Nothing in Visual Basic) to see whether the Mutex could be found. 9. If the Mutex was not found, create the Mutex with the constant string from step 4. 10. If the Mutex was found, close the Mutex variable and exit the application. Your final code might look something like this:

' VB Imports System.Threading Class Program Public Overloads Shared Sub Main() Dim oneMutex As Mutex = Nothing Const MutexName As String = "RUNMEONLYONCE" Try ' Try and open the Mutex oneMutex = Mutex.OpenExisting(MutexName) Catch ex as WaitHandleCannotBeOpenedException ' Cannot open the mutex because it doesn't exist End Try ' Create it if it doesn't exist If oneMutex Is Nothing Then oneMutex = New Mutex(True, MutexName) Else ' Close the mutex and exit the application ' because we can only have one instance oneMutex.Close() Return End If Console.WriteLine("Our Application") Console.Read() End Sub End Class // C# using System.Threading; class Program { static void Main(string[] args) { Mutex oneMutex = null; const string MutexName = "RUNMEONLYONCE";

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try // Try and open the Mutex { oneMutex = Mutex.OpenExisting(MutexName); } catch (WaitHandleCannotBeOpenedException) { // Cannot open the mutex because it doesn't exist } // Create it if it doesn't exist if (oneMutex == null) { oneMutex = new Mutex(true, MutexName); } else { // Close the mutex and exit the application // because we can only have one instance oneMutex.Close(); return; } Console.WriteLine("Our Application"); Console.Read(); } }

11. Build the project, and resolve any errors. Verify that only one instance of the application can be run at once.

To perform atomic math operations, use the Interlock class. To lock data, use the C# lock or the Visual Basic SyncLock syntax. To lock data with a synchronization object, use the Monitor class. To lock data where multiple readers can access data at once but one writer at a time can change data, use a ReaderWriterLock. To synchronize threads across AppDomains or process boundaries, use a Mutex. To throttle threads with a resource-based synchronization object, use a Semaphore. To signal threads across AppDomains or process boundaries, use an Event.

You can use the following questions to test your knowledge of the information in Lesson 2, Sharing Data. The questions are also available on the companion CD if you prefer to review them in electronic form.

Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.

1. Assuming there is not a writer lock in place, how many readers can simultaneously read data with a ReaderWriterLock A. 0 B. 1 C. 10 D. Unlimited 2. Which of the following can be used to synchronize threads across AppDomain and process boundaries (Choose all that apply) A. Monitor class B. Mutex class C. Semaphore class D. C# s lock or Visual Basic s SyncLock keyword

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Through much of the .NET Framework, it is possible to perform tasks in a nonlinear way. By using the Asynchronous Programming Model (APM) defined through the .NET Framework, you can make your applications perform better, be more responsive, and use the resources of the system they are running on to the fullest extent.

After this lesson, you will be able to:

Understand the Asynchronous Programming Model (APM). Use the ThreadPool class. Use the Timer class. Use the IAsyncResult interface to perform asynchronous calls. Understand how exceptions work in the APM.

Asynchronous programming is simply allowing some portions of code to be executed on separate threads. This is referred to as the Asynchronous Programming Model (APM). Throughout the .NET Framework, many classes support using the APM by supplying BeginXXX and EndXXX versions of methods. For example, the FileStream class has a Read method that reads data from the stream. To support the APM model, it also supports BeginRead and EndRead methods. This pattern of using BeginXXX and EndXXX methods allows you to execute methods asynchronously, as shown in the following example:

' VB Dim buffer() As Byte = New Byte(100) {}

Dim filename as String = _ String.Concat(Environment.SystemDirectory, "\\mfc71.pdb") FileStream strm = New FileStream(filename, _ FileMode.Open, FileAccess.Read, FileShare.Read, 1024, _ FileOptions.Asynchronous) ' Make the asynchronous call Dim result As IAsyncResult = _ strm.BeginRead(buffer, 0, buffer.Length, Nothing, Nothing) ' Do some work here while you wait